1. Technical Field
This disclosure relates to computer networking. More specifically, this disclosure relates to methods and apparatus for allocating backbone VLAN identifiers.
2. Related Art
Carrier Ethernet is being heralded as the next major innovation in transport and is being sought after in both the enterprise and the service provider worlds. Conventional Ethernet relies on the principle of Medium Access Control (MAC) address learning and Spanning Tree Protocol (STP) to enable end-to-end communication. Given the absence of any priority information in Ethernet frames and the likelihood of huge spanning trees, the IEEE (Institute of Electrical and Electronic Engineers) 802.1Q standard was introduced to allow an Ethernet domain to be subdivided into smaller broadcast domains called VLANs (Virtual Local Area Networks). A VLAN tag can support 8 QoS (Quality of Service) levels. A single VLAN can have 12-bits to denote the VLAN identifier (VID) resulting in 4096 (effectively 4094 with 2 reserved) tags, and 3-bits to denote 8 priority levels. Since the VID is only 12 bits, the service provider network can only support 4094 unique service instances.
Customers connected to a service provider 802.1Q network may also have their own VLANs (within customer premises). To overcome the limited size of the VID, some networks use stacked VLAN tags—one tag for the customer (also known as the CTAG) and one for the service provider (also known as the STAG). The IEEE 802.1ad standard, which is also known as the Q-in-Q standard, specifies how tags can be stacked.
Although Q-in-Q helps solve the problem of demarcation between customers and providers VLANs, the problem of security from the service provider's perspective still persists as the provider has to learn the customer's MAC address, thus exposing the providers' edge and core bridges to potential MAC security violations.
To facilitate proper demarcation between the customer and the provider, the IEEE 802.1ah or provider-backbone-bridging standard was introduced, which allows a Q-in-Q frame that includes a CTAG from the customer and an STAG from the service provider to be fully encapsulated in a provider backbone bridged frame with a Backbone Service Instance ID (I-SID), provider network specific VLAN-ID (also known as a backbone VLAN identifier or BVID) and Backbone MAC (BMAC).